UNIVERSIT . . AT BONN Physikalisches Institut - Prof. Dr. Norbert ...

7.1. Dynamic range 95
7.1.1 Counter dynamic range
Fig. 7.1 showed the characteristic dynamic range of the CIX 0.2 system. In order to
generalize this result, it has to be analyzed with respect to the theoretically expected
maximum count rate, the impact of the detector bias on the maximum rate and potential
limitations of the counter design.
Maximum count rate
The previous section stated that at 10 keV threshold and maximum feedback settings
(IF b = 91 nA), the system’s maximum count rate lies at 3.3 Mcps. This value was measured
at a tube voltage of 90 keV, which, according to section 6.4.3, corresponds to an average
measured photon energy of approximately 29 keV. A direct crosscheck of this result with
the internal charge injection circuits of CIX 0.2 is not possible because the average pulse
size of 29 keV lies below the minimum input charge of 1.1 fC (32 keV) that the test circuits
can deliver (see section 4.2.1). Nevertheless, the expected maximum count rate at 29 keV
pulse size can be extrapolated from the maximum count rate measurements presented in
section 4.3.1. Fig. 4.8 showed the maximum count rate for different pulse sizes at two
different threshold settings. Extrapolating the curve at 10 keV threshold setting to a
pulse size of 29 keV gives a maximum pulser rate of (8.8 ± 0.2) MHz. However, as the
time interval between individual photons in the X-ray beam is not constant but rather
Poisson-distributed, the value of 8.8 MHz is too large. Section 3.3.1 stated that the count
rate behavior of a paralyzable counter, if exposed to monoenergetic Poisson-distributed
pulses, is given by (3.2). Replacing the incident photon spectrum by a Poisson-distributed
series of monoenergetic photons at exactly the spectrum’s average energy yields:
N Max
Meas = (8.8 ± 0.2) MHz · e −1 = (3.24 ± 0.08) Mcps (7.1)
As the X-ray measurements yielded a maximum rate of (3.3 ± 0.13) Mcps, this shows that
the electrical test and the X-ray measurements are in excellent agreement.
Bias-dependence
Besides the discriminator setting and the spectral distribution of the input pulses, the
maximum count rate in a direct converting semiconductor sensor is also influenced by the
applied bias. Hence, the maximum count rate of different CIX 0.2 modules equipped with
CdTe and CdZnTe sensors was evaluated at various bias settings. Note that the very low
X-ray absorption probability of Si does not allow a similar measurement for the Si module
as the ASIC’s maximum count rate can not be reached with the available X-ray tube.
Therefore Fig. 7.2 only contains the results for the Cd-based sensor materials.
The measurements show that at low bias values the maximum count rate increases with
the bias voltage in all sensor samples. This is a direct consequence of the higher electric
field inside the sensor, which reduces the transit time of the generated charge carriers and
therefore the pulse width and the comparator dead time. In the case of the 1 mm thick
CdTe sample, the maximum count rates are comparable to those of CZT04. However, unlike
CdZnTe the optimum bias value is not equal to the highest safely achievable voltage
across the detector. Instead, a bias of 300 V offers the highest count rate capability. The
reason for this is the photon flux-dependent current, which manifests itself in the CdTe
samples and causes a baseline shift at the preamplifier output (see sections 6.1.3).
In addition, Fig. 7.2 also illustrates that the performance among the CdZnTe samples
varies greatly. The previous section showed that the maximum count rate of the CZT04